The present invention relates to a signal processing circuit for the demodulation of amplitude-modulated signals, like they occur, for example, in RFID systems.
The amplitude shift keying (ASK) is a digital modulation type which has found many areas of use due to its low-effort signal processing. It is, for example, used in RFID systems (radio frequency identification), which enable a wireless identification of a transponder and a data transmission. Further areas of use of the amplitude shift keying may, for example, be found in the area of radio clocks where a carrier signal transmits current time and datum information for a time synchronization. A further example are so-called beacons for a location determination. Here, a carrier signal is modulated with a continuous tone in the audio field for an easier identification, the audio tone itself is again correspondingly modulated (sampled) according to a desired Morse code, so that via the aimed at overall system both the direction and also the identification of the transmitter itself is enabled. The so-called on-off keying (OOK) can be mentioned as the simplest variant of amplitude shift keying. In this method, a carrier signal is switched on or off, respectively, to transmit a binary “1” or a binary “0”.
The envelope of an ASK-modulated signal is illustrated at the top of FIG. 4. FIG. 4 shows two signal courses. The top signal course UASK shows the envelope of an ASK-modulated signal. The bottom signal course shows a signal UDEM, which is extracted when demodulating from the signal UASK. Here, threshold value decisions are made, which means that the signal UASK is evaluated regarding thresholds designated by UTHR1 and UTHR2 in FIG. 4. The demodulator now decides for a signal value U2 if a falling signal edge in the signal UASK falls low of the threshold UTHR1, as it is, for example, illustrated in FIG. 4 at a first transition. If the signal UASK exceeds a second threshold UTHR2 in a rising signal edge, then the demodulator decides for the value U1, as it is indicated as an example in FIG. 4 at the second rising signal edge of the signal UASK. Two exemplary signal values for U1 and U2 are indicated at the bottom in FIG. 4 for clarification. It is to be noted, that the points of time of level changes in the demodulated signal UDEM depend on the edge steepness of the envelope of the ASK-modulated signal UASK. The flatter, for example, a falling signal edge in the signal UASK is, the later the threshold UTHR1 is fallen short of, and for a rising signal edge the same holds true with regard to exceeding the second threshold UTHR2, and the later consequently a change of the level takes place in the demodulated signal UDEM.
In particular when the course of the envelope UASK comprises different signal edges with different steepnesses, additionally a corruption of the symbol durations in the demodulated signal UDEM results. In FIG. 4 this is illustrated as an example. The signal course of UASK shows a flatly falling input edge and a steeper rising output edge in the area marked by dashed lines. The first signal level change in the demodulated signal UDEM takes place relatively late due to the flatly falling signal edge, whereas the second level change in the demodulated signal UDEM takes place relatively early, as the rising signal edge is steeper and consequently UTHR2 is exceeded earlier. The result is that the duration of time the signal UDEM remains on the level U2, as it is designated in FIG. 4 by ΔT2′, is shorter than the duration of the original pulse, designated by ΔT2 in FIG. 4, which modulated the carrier in the transmitter. If the signal UDEM is supplied to such a detector, then, depending on the temporal corruption in relation to the symbol duration, errors may result. Errors of this type are frequently observed in connection with RFID systems. The ASK signals defined in the standard ISO 1443 combined with the tolerances of a PICC (proximity integrated chip card) applied with RFID systems lead to a high error-proneness in the demodulation method illustrated in FIG. 4.